The band structure for single crystal silicon exhibits a conduction band minimum which does not have the same crystal momentum as the valence band maximum, yielding an indirect gap. Therefore, in silicon, radiative recombination can only take place with the assistance of a phonon making such transitions inefficient. This has prevented silicon from being used as a solid state source of light, unlike group III-V semiconductors which have a direct gap at the center of the Brillouin zone. A review of these materials properties can be found in S. M. Sze, Physics of Semiconductor Devices, 2nd. Edition (New York: John Wiley & Sons, 1981). The discovery of photoluminescence in porous silicon has therefore generated a new optoelectronic material for study. A selected review of the fabrication techniques and properties of porous silicon can be found in the articles entitled: "Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers" by L. T. Canham, Appl. Phys. Lett., 57, 1046 (1990); "Visible light emission due to quantum size effects in highly porous crystalline silicon" by A. G. Cullis et al., Nature, 353, 335 (1991); "Visible luminescence from silicon wafers subjected to stain etches" by R. W. Fathauer et al., Appl. Phys. Lett., 60, 995 (1992); "Demonstration of photoluminescence in nonanodized silicon" by J. Sarathy et al., Appl. Phys. Lett., 60, 1532 (1992); and "Photoluminescent thin-film porous silicon on sapphire", by W. B. Dubbelday et al., Appl. Phys. Lett., 62, 1694 (1993).
Porous silicon is formed using electrochemical etching, photochemical etching or stain etching of bulk silicon or silicon-on-sapphire (SOS) wafers as described in the above references. The substrate may be suitably patterned lithographically prior to the etch to define device structures or confine the region exposed to the etch solution. The typical emission spectrum of porous silicon is in the red, orange and yellow region (nominally 500 to 750 nm) although green and blue emission has also been demonstrated. Blue shift of the peak emission wavelength has been shown by increased oxidation and etching of the porous silicon as described in "Control of porous Si photoluminescence through dry oxidation" by S. Shih et al., Appl. Phys. Lett., 60, 833 (1992) and in "Large blue shift of light emitting porous silicon by boiling water treatment" by X. Y. Hou et al., Appl. Phys. Lett., 62, 1097 (1993). These references teach ways to control the wavelength of the emitted light from the porous silicon, but do not teach ways in which the intensity of the emitted light (photoluminescence or electroluminescence) can be controlled. At this time the light emitting mechanism is not fully understood. The scientific controversy surrounding the detailed physical mechanism behind the light emission has not, however, hindered the ability to fabricate porous silicon layers and useful light emitting devices using this technology as described in "Visible electroluminescence from porous silicon" by N. Koshida et al., Appl. Phys. Lett., 60, 347 (1992); "New Results on Electroluminescence from Porous Silicon" by P. Steiner et al., in Microcrystalline Semiconductors: Materials Science & Devices, Materials Research Society Proceedings, 283, 343 (1993) and in "Current injection mechanism for porous-silicon transparent surface light-emitting diodes" by H. P. Maruska et al., Appl. Phys. Lett. 61, 1338 (1992 ).
The use of ion irradiation to quench the light emission from porous silicon has been reported in "Ion-irradiation control of photoluminescence from porous silicon" by J. C. Barbour et al., Appl. Phys. Lett., 59, 2088 (1991). In this report, the authors teach of high energy ion irradiation (24 MeV Cl.sup.+5 ions and 250 keV Ne.sup.+ ions) of a previously formed porous silicon region. The photoluminescence intensity of the ion irradiated porous silicon region can be reduced (at 0.012 eV/atom) and effectively turned-off (at 0.12 eV/atom) by low levels of damage. The amount of damage required to amorphize silicon is greater than 12 eV/atom, which is roughly ten times the value reported to completely quench the light emission. Due to the low levels of damage required to totally quench the photoluminescence using the teachings of the prior art, the processing window for irradiation doses is small which limits the degree over which the intensity can be lowered using this technique. Thus, in accordance with this inventive concept a need has been recognized for alternative techniques which are therefore desired to selectively and reproducibly decrease the intensity in predetermined areas using microelectronic fabrication techniques. Furthermore, the emergence of light emitting porous silicon and porous silicon devices has to date been limited to moderately low light levels. Methods of increasing the intensity of the emitted light are needed for most commercial and technological applications such as displays, light emitting diodes (LED's), optical interconnects and optoelectronic circuits. In accordance with this inventive concept a continuing need has been recognized in the state of the art for a method of controlling the photoemission of porous silicon using ion implantation.